So many problems of software are problems of classification: what is the best way to cluster concepts/structures/behaviors so as to aid understandability, form crisp abstractions, and yield a balanced distribution of responsibilities? So it has been in classifying the systems under study in the Handbook. If you've been following the saga of my work here, you'll see that I've grouped the 100 systems in the handbook in one of nineteen genres. I will be the first to admit that there are some imperfections in my grouping (is an ATM a device or a financial system?) but so far I think it's good enough. I've had a few readers write with questions and observations (thank you!) and as a result have jiggled around a few of the groupings. Today I finished writing up a description of each genre which you can read here (assuming you are logged in to the site).
Bill included in his posting (mentioned in my previous entry) the book Women, Fire, and Dangerous Things, a delightful romp through the problems of classification. If you want an even weightier tome, check out Duda's Pattern Classification. A fun and much lighter read may be found in Kipfer's The Order of Things.